A long chain branching (LCB) polymer is a polymer containing one or more side chain branches whose length is comparable to or longer than a critical entanglement length. Compared with a linear polymer having the same molecular weight, a long chain branched polymer shows high shear sensitivity, zero shear viscosity, melt elasticity, and high impact strength (Graessley, Acc. Chem. Res. 1977, 10, 332; Bersted, et al. J. Applied Polym. Sci. 1981, 26, 1001; Roovers, Macromolecules 1991, 24, 5895). LCB polymers exhibit higher viscosities at low shear rates and lower viscosities at high shear rates. Shear thinning is advantageous in polymer processing, such as under high shear conditions. Further, high melt strength, that is, increasing resistance to stretching during elongation of the molten material, is a desirable mechanical property which is important in thermoforming, extrusion coating, and blow molding processes involving predominately elongational flows.
Polyolefins produced with single-site catalysts, such as metallocene and non-metallocene catalyst systems, generally have relatively narrow molecular weight distribution characterized by a polydispersity index of about 2. This narrow distribution leads to superior mechanical properties, but worsens processibility because of the lack of shear thinning. On the other hand, polyolefins produced with traditional Ziegler-Natta catalysts, which contain multiple active sites, show broad molecular weight distribution with good processibility characteristics, such as shear thinning, but with undesirable mechanical properties.
Polypropylene is a commonly used polyolefin, nearly ubiquitous in modern industrial use. Polypropylene is particularly desirable as a high quality plastic because it can be purified to a high degree and it is resistant to microbial growth, making it an excellent material for use in medical applications and in the semiconductor industry. Further, as a lightweight, chemical and heat resistant material, polypropylene is useful in manufacture of diverse packaging materials, textiles and consumer items. However, commercial polypropylene (PP) products, normally isotactic, semi-crystalline thermoplastics, prepared by Ziegler-Natta or metallocene catalysts, have a predominantly linear molecular structure. Although linear PP polymers have many desirable physical properties, they show a variety of melt processing shortcomings, especially the metallocene-prepared ones having narrow molecular weight distributions. The low melt strength causes local thinning in melt thermoforming, relative weakness in large-part blow molding, the onset of edge weave during high speed extrusion coating of paper or other substrates, and flow instabilities in coextrusion of laminate structures. As a result, PP has been limited in some end-use fabrications, for example, extrusion coating, blow molding, profile extrusion, and thermoforming.
One way to improve the processing deficiency of such polymers is to introduce long chain branches to polymers. However, there are a number of problems remaining to be overcome in this area. For example, an in situ, or one-pot, process for LCB polymer synthesis is desirable to address economic and environmental concerns regarding polymer synthesis.
Furthermore, despite intense interest and many research attempts, so far there is no commercially viable process for preparing long chain branched polypropylene (LCBPP).
In a direct polymerization process, one major difficulty of in situ preparing LCBPP polymers is due to the complicated PP macromonomer structures. There are two possible monomer insertion modes (including 1,2- and 2,1-insertions) and multiple chain termination mechanisms that can lead to polypropylene with various chain ends (Weng, et al. Macromol. Rapid Commun. 2000, 21, 1103), while only the vinyl chain end is effective for LCB formation. Furthermore, the preparation of the most important isotactic polypropylene requires iso-specific catalysts, such as rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO, which have limited special opening at the active site for incorporating macromonomers. Therefore, it is extremely difficult to find a catalyst system that can accommodate all the requirements, namely in situ formation of a significant amount of vinyl-terminated PP macromonomers and further incorporation of macromonomers into LCBPP structure.
In addition, under some reaction conditions, a small portion of the incorporated diolefin units might engage double enchainment, and the increase of cross-over structures in the polymer results in unprocessible (crosslinked) polymer network.
Thus, there is a continuing need for LCB polymers as well as methods and reagents for use in their synthesis.